U.S. Pat. No. 7,604,652 to Arnin describes a spinal prosthesis that includes first and second spinal attachment members attachable to first and second posterior portions of spinal structure, respectively. The first and second posterior portions are adjacent superiorly-inferiorly to one another. A connector element is attached to the first spinal attachment member. The second spinal attachment member includes an interface portion that passes through an elongate aperture formed in the connector element so as to permit rotational and translational movement of the second spinal attachment member with respect to the connector element.
The elongate aperture defines limits of movement of the second spinal attachment member with respect to the connector element. The interface portion of the second spinal attachment member is integrally formed with abutments spaced from the end faces of the elongate aperture. The abutments define limits of movement of the interface portion. The end faces and the abutments may have curved contours that mate with each other.
Since the abutments are part of the interface portion of the second spinal attachment member, the position of the abutments is basically the same for all patients. This is disadvantageous because the anatomy of patients can vary widely; if the abutments are not placed correctly with respect to the particular anatomy, the abutments can lose their effectiveness, or worse, can interfere with the correct functioning of the prosthesis.
Another disadvantage is that the spinous process can interfere with the abutment members, which means a laminectomy must be performed to remove the spinous process.